According to US Government regulations, a network carrying unencrypted classified data must be protected by a Protected Distribution System (PDS). One form of PDS is the Alarmed Carrier, which is a system by which a conveyance or carrier is alarmed by a device for detecting intrusions and attempted intrusions.
An example of an arrangement of this type is shown in U.S. Pat. No. 7,706,641 issued Apr. 27, 2010 by the present Applicants, as well as application Ser. No. 14/144,882 filed Dec. 31, 2013 by the present applicants now pending and entitled Network Alarm System Utilizing a Single Sensing Fiber, the disclosure of which is incorporated herein by reference.
Another example of an arrangement of this type is shown in U.S. Pat. No. 7,142,737 issued Nov. 28, 2006 by the present Applicants, the disclosure of which is incorporated herein by reference. This discloses a method of detecting a potential intrusion event by sensing changes in polarization and uses Fresnel reflections and Rayleigh backscattering in time sliced data to locate the intrusion event at its position along a fiber.
Reference is also made to application Ser. No. 14/145,121 filed Dec. 31, 2013 by the present applicants now pending and entitled An Alarm System For An Optical Network and which relates to a method in which there is provided at each of said end user terminals a monitor signal receiver system for detection and conversion of data from the analysis into a signal and for transmitting the signal from each of the user end terminals back to the monitor system, the disclosure of which is incorporated herein by reference.
The above patent describes that some or all of the optical fibers of a single-mode or multi-mode cable are monitored for intrusion by transmitting through the fibers a signal which can be analyzed for changes in its characteristics which are indicative of movement as a prelude to an intrusion event.
Data can be stolen from an optical fiber by removing the jacket and installing a tap device on the fiber. Optical fiber intrusion detection systems of the type described above detect when a fiber cable is being subjected to vibration and handling that would be typical of an intrusion attempt. The system reports the intrusion attempt to the cable owner or other responsible individual so that the cable can be inspected and the threat removed.
The fiber intrusion detection system works by transmitting a monitoring signal through a fiber. Disturbances on the fiber cause the monitoring signal to be modulated. At an end of the loop, the modulated signal is received, digitized and processed, and alarms are raised when an intrusion is detected.
Historically, alarmed carriers required two fibers for monitoring-classically one carrying the laser signal away from the monitoring device, and one carrying the alarm signal back, these joined at the far end (known as “looping back”).
Recent advances in technology have seen the introduction of the Passive Optical Network (PON). A passive optical network (PON) is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically in the range 16-128. The PON is a system which delivers bidirectional communication over a single fiber by use of separation of send and receive by utilizing separate wavelengths. A PON consists of an optical line terminal (OLT) at the service provider's central office and a number of optical network terminals (ONTs) near the end users. A PON reduces the amount of fiber and central office equipment required compared with point-to-point or other legacy network architectures.